1. Field of the Invention
This invention is related to a substrate for epitaxy used to make opto-electric devices such as LED and LD or electric devices such as MOSFET.
2. Discussion of the Related Art
The currently used substrates for a vapor phase epitaxy typically include Sapphire, SiC, GaAs and Si and the resulting epitaxy layers grown on the substrate have been still in the unfavorably high dislocation density of 109/cm2. Therefore, there have been proposed an application of ELOG-type structures to the substrate, which allows to decrease the dislocation density down to 106/cm2, but that density is still too high to ensure the correct functioning of many electronic and opto-electronic devices, particularly high power semiconductor lasers. Further, there is another problem that the resulting substrates have a decreased epitaxy face area because of the ELOG structure. Accordingly, we have proposed the other processes for obtaining mono-crystalline gallium-containing nitrides, For example, Polish patent application No. P-347918, where there is proposed a method of obtaining bulk mono-crystalline nitrides, represented by gallium nitride, through their re-crystallization from supercritical ammonia-containing solution. A characteristic feature of the bulk mono-crystalline gallium nitrides obtained using the afore-mentioned supercritical ammonia-containing solution is their low dislocation density (in the case of the bulk GaN: 104/cm2). It is, however, obtained at a low growth rate, in fact many times lower than the growth rate used in the growth methods from the gaseous phase.
1) The inventors of this invention have further discovered from their sharp researches that it is possible to significantly lower the dislocation density of the gaseous or vapor phase epitaxy layer without the ELOG structure if such a gaseous phase growth process is carried out on the surface of a substrate of bulk mono-crystalline gallium-containing nitride made by means of crystallization from supercritical ammonia-containing solution, and also to keep all the main surface of the substrate as the complete Ga-polar face for further epitaxy process of making opto-electric or electric devices. This is quite different from the ELOG-type substrate.
Furthermore, 2) the inventors have discovered that A-axis direction growth of GaN is 4 times or more faster than that of C-axis direction in a supercritical ammono method containing at least one alkali metals and get a mono-crystal nitride substrate having a diameter of 1 inch or more provided with a good quality such as C-axis orientation perpendicular to the C-plane extending at least 30 μm, preferably 50 μm or more. This is also quite different from the ELOG-type substrate.
According to the above findings 1) and 2) of the inventors, if an GaN layer is grown by means of HVPE on a mono-crystal nitride substrate made by the supercritical ammono method to have a thickness of 100 μm or more, preferably 150 μm or more and is sliced off from the mono-crystal nitride substrate, the resulting HVPE GaN have a dislocation density of 5×105/cm2 or less, preferably of 105/cm2, more preferably 104/cm2 or less and one of the surface substantially consists of Ga-polar face and is better than any ELOG GaN substrate.